With the advent of digital computing, including digital photography, there has been an explosion in the amount of digital images, such as photographs and video, that are generated and transmitted by electronic imaging equipment, such as digital cameras, camcorders, copiers, scanners, PDAs (Personal Digital Assistant), cell-phones, and the like. A great deal of such images are created by amateur photographers, using imaging equipment with limited dynamic range (difference between highest and lowest values of a variable quantity such as light intensity, frequency, and the like,) under unsuitable lighting conditions, and/or a combination thereof, resulting in sub-optimal images having various visual deficiencies such as overly dark or bright segments or otherwise bad lighting, blur, noise, poor color, graininess, distortions, and the like.
One of the image quality shortcomings in many images, such as photographs, is that when a poorly lit scene is captured, some of the original scene detail is lost in the regions of the image that are dark (shadows) and/or bright (highlights) compared with other regions of the image. This is particularly notable when a scene contains mixed lighting where there are combinations of shadows, highlights, and medium regions that are captured in a single image. This shortcoming may be caused by the limited dynamic range and/or imperfect exposure settings of the image capture device, among other causes.
A variety of techniques have been used to address image deficiencies. One technique to enhance an image is to adjust the lighting of the original scene so that there is limited dynamic range in the scene that is to be captured, for example, by eliminating overly dark and/or overly bright areas in the scene. Other than in a studio where lighting may be adjusted professionally, adjusting the lighting of a scene may be difficult at best and impossible at worst because of lack of control of the scene, equipment, knowledge, or any combination thereof.
Another technique to improve an image is to capture the scene with a device with better dynamic range, focus, light metering, various compensation schemes, and the like. This technique also presents limited opportunities for improvement due to unavailability of suitable equipment, time constraints, lack of control over the scene or equipment, and the like. Additionally, the quality of a captured image is generally not known until after it has been captured.
Still another technique to enhance a captured image is to post-process the captured image to enhance the detail of the overly dark and/or bright regions of the image and/or make other enhancements. Processing an image after image acquisition is usually possible and desirable in most circumstances because the constraints for post-processing are fewer. However, most conventional image processing methods are based on image processing algorithms with a number of settings or parameters that are preset to default values, are adjusted by the user in a trial and error process, or otherwise are set without being directly based on the characteristics and attributes of the original image. For example, a user controlled software package such as Photoshop® may be used to enhance captured pictures by iteratively tuning the available parameters to converge on an acceptable solution (enhanced image). This process can be a complex and time consuming task and may yield less than optimal results. Such conventional methods for setting the parameters of image processing algorithms are generally sub-optimal for a particular image and may introduce other undesirable visual artifacts in the enhanced image.